1. Field of the Invention
The present invention relates to cutting tools such as hobbing cutters, end mills and so on having cutting edges made of high speed tool steels.
2. Description of the Related Art
High speed tool steels and cemented carbides are used as typical conventional cutting tool materials of cutting tools such as hobbing cutters, end mills and so on.
High speed tool steels for cutting tools containing more than 2% of Vanadium (V) are exploited as cutting tool materials having a high hardness, wear resistance and a high heat resistance.
Cemented carbides, on the other hand, are very hard and exhibit both excellent wear resistance and high heat resistance. However, because of the cemented carbides are frangible and of high material costs as well as high machining costs, and further, the cemented carbides have inferior in both grindability and anti-shock resistance compared with the high speed tool steels, the cemented carbides are not presently in wide practical use as cutting tool materials for reasons of a lack of credibility in quality and high material costs as well as high machining costs. Therefore most hobbing cutters which perform interrupted cuttings are made of high speed tool steels, and only small parts thereof are made of cemented carbides.
High speed tool steels for use for such cutting tools usually contain more than 2% of Vanadium (V) to enlarge the carbide grain size of hard MC-type carbide grains mainly consisting of Vanadium alloys. However, conventional high speed tool steels produced by melting which contain more than 3% of V, result that the more amount of V increase, the more become liable to generate crystallized coarse or elongated angled MC-type carbide grains, deteriorating both grindability as well as toughness, and become liable to cause a problem of a chipping or fracture of the tool cutting edge starting from one of the MC-type carbide grains.
Also, conventional high speed tool steels produced by sintering powder disclosed, for instance, in U.S. Pat. No. 5,252,119, are liable to generate too fine grain particles of MC-type carbides which result insufficient toughness causing a problem of quick or large wear of the cutting edge and shorten tool life despite of using high cost materials.
In wet-cutting of hobs using cutting oil, although the whole surfaces of the hob cutting edges are initially coated with Ti- or Ti-alloy ceramic coating films, after re-ground of the hob cutting edges are performed, the coating films deposited on the cutting faces, or rake faces, are removed away, and there are required both heat- and wear-resistances of the high speed tool steel materials themselves of the hobs.
On the other hand, in dry-cutting of hobs performed without the supply of cutting oil, the hob materials must have high hardness, sufficient toughness to resist shocks by interrupt cuttings of hobs and heat resistance. In addition, the surfaces of the cutting edges must deposit coating films having wear-, heat-, and acid-resistances as well as lubricative properties to prevent wear, chipping and fracture thereof. Further, the hob materials are required to have a good affinity with the above coating films.